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United States Patent |
5,114,645
|
Niiler
,   et al.
|
May 19, 1992
|
Fabrication of ceramics by shock compaction of materials prepared by
combustion synthesis
Abstract
A method and apparatus for the production of high density, monolithic
cerc materials. A mixed powder such as Titanium-Carbon is first subjected
to uniaxial pressure to form a green compact. The compact is positioned in
a vented mould between two steel plates, reacted by a combustion synthesis
process called SHS and then subjected to an explosively generated
shockwave by an explosive charge positioned on the upper steel plate,
which generates high pressures to compress the hot, porous ceramic to form
high purity ceramic of more than 90% theoretical densities.
Inventors:
|
Niiler; Andrus (Bel Air, MD);
Moss; Gerald L. (Newark, DE);
Eichelberger; Robert J. (Bel Air, MD)
|
Assignee:
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The United States of America as represented by the Secretary of the Army (Washington, DC)
|
Appl. No.:
|
296999 |
Filed:
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December 6, 1988 |
Current U.S. Class: |
264/84; 264/80 |
Intern'l Class: |
C04B 033/32 |
Field of Search: |
264/84,80
|
References Cited
U.S. Patent Documents
4655830 | Apr., 1987 | Akashi et al. | 264/84.
|
Primary Examiner: Derrington; James
Attorney, Agent or Firm: Elbaum; Saul, Roberto; Muzio B.
Goverment Interests
RIGHTS OF THE GOVERNMENT
The invention described herein my be manufactured, used, and licensed by or
for the Government for Governmental purposes without the payment to us of
any royalties thereon.
Claims
We claim:
1. A method for the fabrication of a high density, monolithic ceramic
material comprising the steps of:
subjecting a mixed powder to a uniaxial pressure to form a green compact,
subjecting the compact to a solid combustion reaction employing
self-propagating high temperature synthesis to form hot, porous ceramic,
subjecting the hot ceramic while at a temperature above its ductile brittle
transition temperature to an explosively generated shockwave to generate
high pressures to compress the ceramic, and
slowly cooling the material.
2. A method in accordance with claim 1, wherein the compressed ceramic is
heated to a temperature of about 2400.degree. C. for a period of one hour
and cooled.
3. A method in accordance with claim 1, wherein the mixed powder is
selected from the group consisting of Titanium-Carbon, and Titanium-Boron.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a method and apparatus for the production of
density monolithic, ceramic material, comprising subjecting elemental
powders to a combustion synthesis process in a novel system to produce a
porous material, and compacting the ceramic to high density by pressure
wave initiated by a high explosive.
2. Background Information
High-technology, structural ceramics are becoming utilized in ever larger
range of applications which require light-weight, high-temperature,
high-performance materials. These ceramics are typically the borides,
carbides, nitrides and oxides of a variety of metals, and are fabricated
in such a way as to eliminate most, if not all, porosity and impurities
from the bulk. The production of these ceramics, however, involves
complicated processes which, in general, use expensive starting materials,
are very energy and labor intensive and so result in very high final
product costs.
The most commonly used commercial process for fabricating these ceramics is
hot pressing of the ceramic powder into the shape of a disc, rod or plate.
Green (unfired) compacts of previously processed ceramic powders are
placed into a die in a high temperature furnace and pressed with uniaxial
pressure under inert gas atmosphere. While the sample is at high
temperature, but well below the melting point of the ceramic, sintering
(or solid welding) takes place during which the powder particles coalesce
into a solid body. The length of time required for the whole body to be
sintered may vary from a few to tens of hours. Since temperatures of
1200.degree. C. to 1600.degree. C. are quite common, these hot pressing
operations can be very expensive from the energy use standpoint. In
addition, the high temperature presses with atmospheric control represent
a very high capital cost which increases in proportion to the volume of
the sample to be processed.
Another means by which high-technology ceramics are fabricated is by Hot
Isostatic Pressing (HIP). The major difference between HIP and hot
pressing is that in HIP, isostatic pressure is exerted on the ceramic body
during the high temperature cycle rather than uniaxial pressure. The
sample to be fabricated is enclosed in a metal envelope, usually of
tantalum or stainless steel, cold pressed to the desired shape, and then
heated to temperatures up to 2000.degree. C. under the pressure of a
working fluid, usually an inert gas, at a pressure in excess of 120 MPa.
The advantage of HIP over hot pressing is that complicated shapes can be
produced. In HIP, as in hot pressing, sintering is the mechanism by which
sample consolidation takes place. However, the equipment and green compact
preparation are even more complicated thus raising the product cost
higher.
High-technology ceramic materials can also be produced by a process called
Self-Propagating High-Temperature Synthesis (SHS). This process involves
solid combustion reactions between constituent powders, which are
characterized by very high heats of reaction, i.e, reaction temperatures
of about 3000.degree. C., and reaction front velocities of a few cm/sec.
It is possible to ignite the powder mixtures with a very small amount of
energy at which point the heat of reaction that is released sustains
further reaction until the whole sample has been synthesized. Fabrication
by SHS has a number of distinct advantages over the conventional processes
previously discussed. The fact that it is a high temperature process
produces a self-purging effect, whereby most contaminants are driven from
the sample during the reaction. Since all heat except for the small amount
needed for ignition is supplied by the exothermic reaction, the process is
highly energy efficient. In addition, the process is potentially more
economical than the conventional processes since no high temperature
furnace is needed. Because the product is formed at a temperature usually
exceeding 2000.degree. C., phases and compositions that cannot be formed
at the lower temperatures of conventional processing may be feasible.
The SHS process has been successfully used in a number of applications,
both in the US and abroad. Possibly the greatest successes have been
achieved in the Soviet Union where the manufacturing of ceramic powders
such at TiC, TiB.sub.2, SiC and B.sub.4 N, among others, is now done
commercially. In addition, the Soviets are using SHS to produce tool bits,
dielectric materials, heater elements and high temperature filters. Ir
Japan, as well as the US, the production of materials by the SHS process
has not progressed to the commercial stage as of yet, but significant
applications are being pursued. In the US, applications have been limited
to the use of SHS as a source of heat in thermal batteries and aerosol
dispersal and as a source of the IR signal in TOW missiles. The thermite
reactions that are widely used for field welding of steel are probably the
most common application of he SHS reaction principle.
However, there have been serious technical problems associated with the
fabrication of high density products by this method. The first is the fact
that when mixed powders are reacted by SHS, the product generally exhibits
as much as 50% porosity, whereas as little as 0.5% porosity in ceramic
materials can be detrimental to performance. The second problem is the
cracking of the sample during processing. Gases formed from impurities on
the powders which when driven off at the high temperatures, form channels
in the sample which become crack initiation sites. Cracking is also caused
by the thermal shock to the sample as it cools down from over 2000.degree.
C. to ambient in a very short time. If the sample is mechanically loaded
in order to increase its density, and its temperature at the time of
loading is below the ductile-brittle transition temperature, the internal
stresses introduced during such loading may initiate cracking. Another
problem is related to the bonding of final product grains to each other.
The sample performance depends not only on the absence of porosity but
also on the integrity of the inter-granular bonds. The long times at high
temperatures needed for sintering action to take place (and its
concomitant strong intergranular bonding) is not available for the SHS
process since sintering temperatures are sustained for only a few minutes.
The final problem is the difficulty in predicting the product properties
and synthesis process behavior from the initial powder and compact
properties, the initial geometry and ignition parameters.
Several experiments which utilize the SHS principle to achieve high density
products of TiC and TiB.sub.2 have been attempted in US laboratories.
Titanium/Boron and Titanium/Carbon powder mixtures have been heated to
ignition inside graphite dies under uniaxial pressure placed inside high
temperature furnaces. Densities of about 95% have been achieved for the
reacted product. The Ti/B and Ti/C powder mixtures placed inside insulated
steel dies have been ignited by tungsten filaments or other small energy
sources and reacted. After reaction, the porous product has been compacted
by uniaxial pressing in a hydraulic press resulting in densities of about
88%. Ti/C mixtures, encased in insulated tubes have been ignited and
continuously compacted in a rolling mill immediately following the passage
of the reaction front. Small areas of high density have been achieved by
this method.
Explosive consolidation technology has also been used in ceramics
processing. An explosively generated shock wave has been used to attempt
both ignition and compaction of Ti/B powder mixtures held in strong
containment. The results of this experiment have shown complete SHS
reaction of the powders but little or no compaction of the product
TiB.sub.2.
This explosives technology has been applied to powdered metals resulting in
successful consolidation of both cylindrical and plate forms. Factors
affecting the consolidation are the pressure attained, load duration, and
the material being compacted. A cylindrically converging system has been
used almost invariably to consolidate high melting point ceramic powders.
Starting with Al.sub.2 O.sub.3 powders at room temperature, explosively
driven cylindrical compactions have produced material with reasonably
pore-free local regions. However, the degree of compaction varies with
radius and is sometimes further disrupted by thin spiraled regions of
micro-cracked material that occurs because the compaction is spatially
nonuniform.
The generally observed result is that large, crack-free specimens of hard,
high melting point ceramics cannot be prepared by explosively compacting
ceramic powders from the room temperature state. Aluminum nitride is an
exception which is readily consolidated because, it is believed, it
becomes plastic at high pressure. Partially successful explosive
consolidation of ceramic powders has been accomplished by preheating the
powders before compaction. Both cylindrical and flat plate samples, crack
free and of high density have been made in this way.
DISCLOSURE OF INVENTION
The basic concept of this invention is to make a ceramic body of TiC,
TiB.sub.2 or any other ceramic which can be reacted by the SHS process,
apply no external heat to the system except for what is needed to ignite
the SHS reaction, hold the sample in minimal insulation to keep it from
cooling too quickly, and while this temperature is above the
ductile-brittle transition temperature, compact it to full density with a
high pressure wave from an explosive charge. This concept applies to any
and all shapes which can be made into green compacts and also have the
geometrical symmetry to which a shock wave can be applied. There is no
limit to the length and width of plates that can be processed under the
concept covered by this invention. The thickness attainable depends on the
amount of explosive used.
The first step in the process is preparation of the powders and making a
green compact. Although the purity of the powders is not limited, it is
advisable to use the highest purity available, since any volatile that is
found on the powder will be driven off, rather violently, by the very high
temperatures of the reacting sample. This action of the impurity gases
leaving the sample may disrupt the sample sufficiently to introduce
sizeable cracking which may not fuse under any pressure conditions. The
optimum powder size has been found to be about -325 mesh. Larger size
powders do not react very well in some systems and smaller sizes have such
large surface areas that special precautions must be used to prevent
absorption of water vapor and other contaminants. Also, smaller sizes may
react so quickly that the above described disruption due to even small
amounts of volatiles may be a serious problem. The component powders must
be thoroughly mixed and pressed into suitable green compacts. There is no
limit on the size or shape of the compact other than that it must have
symmetry about a plane or axis so as to allow compaction in a direction
perpendicular to that plane or axis. Flat discs and rectangular plates are
the simplest shapes that meet these pressure symmetry conditions. The only
requirement on the pressure at which the green compact is prepared is that
it be high enough to keep the compact from falling apart during subsequent
assembly.
The green compact is now placed into a container in which both the SHS
reaction and explosive compaction can take place. The first requirement on
the container is that it be capable of containing the sample during the
SHS reaction. While the sample is reacting, the movement of volatile gases
out of the reaction zone will tend to break up the sample, sometimes
before the reactions has come to completion. Thus, the container must be
strong enough at the high reaction temperature (2000.degree. C.) to keep
sample pieces from flying away. Mild steel has been found to satisfy this
requirement. The container must also be able to allow the escaping, high
temperature gases to exit. This condition is met by providing vent holes
in the container wall and leaving some space between the sample and the
inside walls of the container to allow for relatively free gas movement.
Another requirement is that the thermal conductivity and heat capacity of
the container be low enough so that the heat generated by the reacting
sample is not drained away to the container walls thus quenching the
reaction. This requirement is met by making the steel container out of a
thin, annular ring and backing this ring with a thermally insulating
material such as plaster. The fact that the plaster is used for the bulk
of the containment vessel satisfies another requirement, namely that the
compressibility of the container by the explosively generated pressure
wave be roughly equivalent to that of the ceramic sample. Finally, a high
temperature, relatively inert material must be placed between the sample
and the steel container to prevent the iron from diffusing into the hot,
reacted sample. Grafoil and Zirconia sheet have been found to satisfy this
condition.
The top and the bottom of the containment vessel are made of hard steel
plates, the bottom being the anvil against which the sample is compressed
and the top being the compression plate which provides the compaction
pressure. A space is left between the bottom of the compression plate and
the top of the green compact. This space is partially filled with the
material used to initiate the SHS reaction in the compact. A thin layer of
insulating material is placed at the bottom of the compression plate and
the top of the anvil plate for additional containment of the heat. A layer
of powdered high explosive is placed on top of the compression plate and
the whole assembly is then placed into a flattened hole at the top of a
pile of sand.
The green compact is remotely ignited and the whole sample is allowed to
react. Completeness of the combustion can be verified by either visual
observation through a periscope or by thermocouples placed at the bottom
of the container vessel. After the synthesis reaction is complete, but
before the temperature drops below the ductile-brittle transition
temperature of the ceramic, detonation of the explosive is initiated. The
explosive is initiated so that detonation wave sweeps across the steel
cover plate and the hot ceramic. The effect of the explosion as it
propagates over the containment system is to drive the steel compression
plate, as a piston, into the hot porous ceramic reaction product and to
compress the ceramic to a high density.
As the hot ceramic is compacted by the explosive load, it is further heated
by the irreversible work done during compaction. This corresponds to the
difference in the pressure-specific volume path during compaction of the
porous ceramic from the path in the same space following consolidation as
the pressure drops from the compressed state to ambient. This irreversible
heating serves to supplement the heating from the SHS reaction and,
thereby, to help maintain the thermal conditions necessary for compaction
and bonding. To prevent the newly consolidated, and still hot, ceramic
sample from undergoing too great a thermal shock, the hot assembly is
covered with sand to keep it from cooling too quickly. Initiating the
explosive so that detonation wave sweeps over the sample is the condition
that allows one to compact samples of unlimited dimensions lateral to the
direction of compaction.
In addition to the SHS reaction and explosive compaction, a post compaction
thermal treatment is included as a part of the process claimed. The state
of the product after compaction is dependent on the temperature attained,
the timing of the compaction after the synthesis reaction, the type of
explosive used, and the rate of cooling. For some conditions, it may be
advantageous to heat the sample after compaction to further enhance
bonding. For example, it has been demonstrated that for one set of
synthesis and compaction conditions, the compacted sample was easily
broken by hand. This sample was subsequently heated to 2400.degree. C.,
held at temperature for one hour, and furnace-cooled. The Knoop hardness,
HK(100 g), of the sample after this treatment was 3180.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram of the assembly showing the sample, container
and explosive charge.
FIG. 2 and FIG. 3 are an illustration of a typical porous charge being
compressed by a sweeping explosive charge.
FIG. 4 is an illustration of the density contours (in 1% of theoretical
density) in the sample at 15.88 midoseconds after line wave initiation.
BEST MODE FOR CARRYING OUT INVENTION
Referring to FIG. 1 of the drawings, green compact 11, 5.2 cm diameter and
2.5 cm thick, of mixed titanium and carbon powders is pressed in a
uniaxial die to a pressure of 137 MPa. The compact composition is a 55/45
molar mixture of -325 mesh Titanium and submicron Carbon black powders.
This compact is placed in a 3.8 cm high by 20 cm by 20 cm plaster block 13
whose center has been cored to a diameter of 6.1 cm. Mild steel ring 15 of
1 mm wall thickness and 3.8 cm high is placed between the compact 11 and
the plaster 13, in contact with the plaster 13. Both the steel ring and
the plaster block have matching vent holes 17 to the outside of the block.
A 0.25 mm thick sheet of Grafoil 19 is placed between steel ring 15 and
the compact, leaving approximately a 3.2 mm space 21 between the Grafoil
and the compact. 1 cm thick, high hardness steel plates 23 and 25 are
attached both to the top and bottom of the block of plaster 13. A 1 mm
thick sheet of Zirconia 27 insulation is inserted in between the bottom
steel plate 25 and the compact 11. On top of the green compact is laid 5
grams of a mixture of loose titanium (-400 mesh, 3.5 g) and boron (5
micron, 1.5 g) powders 31 with an electric match 33 at the center, the
electrical leads 35 from the match being taken out through one of the
holes. Another layer of Zirconia 37 insulation is placed between the top
of the igniter powder 31 and the top steel plate 23. Steel plates 23 and
25 are held in place by plexiglass plates both top and bottom and
connected by thin, brass threaded rod. A plexiglas box to contain the
explosive powder 38 is attached to the top plexiglass plate. FIG. 1 shows
a diagram of this assembly.
This assembly, loaded with a 5 cm layer of amatol explosive powder 38
(about 0.5 KG) is placed on the leveled top of a sand pile. Electric match
33 is ignited remotely by a 45 Volt battery. Burning electric match 33
ignites the loose Ti+B powder 31, which in turn ignites the Ti+C green
compact 11. The gases that are released during the combustion of the
sample escape through vent holes 17 in steel ring 15 and backing plaster
container 13. The heat from the Ti +C SHS reaction is high enough to not
only propagate the reaction through the whole sample, but also to heat the
sample to temperatures in excess of 2000.degree. C. Although the reaction
proceeds to completion in 5 to 10 seconds, the sample temperature stays
above the ductile-brittle transition temperature for TiC (1400.degree. C.)
for several minutes. At about the 10 second mark, or when the temperature
of the reacted sample has equilibrated prior to beginning to decrease,
explosive 38 is initiated so that a detonation wave sweeps over the
sample. This explosion sets compression plate 23 in motion and it, in
turn, applies a high pressure on the hot, reacted TiC sample 11,
compacting it to high density.
A TiC sample fabricated by the above described procedure was found to have
the following properties: Final diameter, 5.5 cm; Final thickness, 1.2 cm;
Core density, 88.7% of theoretical; Hardness, HK (400 g) -1465+/-300;
X-ray diffraction results show only TiC, no free Ti or C.
A second sample, of TiB.sub.2 fabricated in a similar fashion as above but
with the precursor powders being -325 mesh titanium and 0.5 micron
amorphous boron in a 33/67 atomic ratio, was found to have the following
properties: Final diameter, 5.5 cm; Final thickness, 1.2 cm; Core density,
93.8% of theoretical; Hardness, HK (400 g) -2079+/-250; X-ray diffraction
results show only TiB.sub.2 no free Ti or B.
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